1. Field of the Disclosure
The exemplary implementations described herein relate to a current-feedback operational-amplifier based relaxation oscillator as a versatile electronic interface for capacitive and resistive sensors.
2. Description of the Related Art
Oscillators are widely used as electronic interface for sensors. This is attributed to their simplicity and immunity to electromagnetic interference. While harmonic oscillators, widely used for capacitive and inductive sensors, have a very high sensitivity, due to their resonant nature, relaxation oscillators are simpler and less sensitive. This explains the growing interest in designing relaxation oscillations using operational amplifiers, operational transconductance amplifiers, second-generation current-conveyors and current feedback operational amplifiers (CFOAs). Of particular interest here is the relaxation oscillators built around CFOAs. This is attributed to their higher signal bandwidths, greater linearity, wider dynamic range, simple circuitry and low power consumption.
Inspection of the available current-conveyor based relaxation oscillators shows that a Schmitt trigger is required for each circuit implementation. While some implementations use current-in current-out Schmitt triggers others use the input-voltage output-voltage Schmitt trigger shown in FIG. 1 and described in Catalodo et al., (“A Schmitt trigger by means of a CCII+, International Journal of Circuit Theory and Applications, Vol. 23, 1995, pp. 161-165-incorporated herein by reference). Inspection of FIG. 1 shows that this Schmitt trigger circuit requires three resistors and does not have a low-output impedance terminal. Moreover, using this Schmitt trigger circuit a relaxation oscillator was proposed in Almashary et al., (“Current-mode triangular wave generator using CCIIs”, Microelectronics Journal, Vol. 31, 2000, pp. 239-243-incorporated herein by reference) with the resistance R replaced by the internal parasitic resistance rx of the second-generation current conveyor; as shown in FIG. 2. Thus, cascading the relaxation oscillator circuit, based on FIG. 1, for further signal processing, may require an impedance matching circuit.
Furthermore, the relaxation oscillator reported in Almashary et al., (“Current-mode triangular wave generator using CCIIs”, Microelectronics Journal, Vol. 31, 2000, pp. 239-243-incorporated herein by reference) was used as the basis for designing a current-conveyor based relaxation oscillator versatile electronic interface for capacitive and resistive sensors as described in Abuelma'atti et al., (“A current conveyor-based relaxation oscillator as a versatile electronic interface for capacitive and resistive sensors,” International Journal of Electronics, Vol. 92, 2005, pp. 473-477—incorporated herein by reference). In the interface circuit of the parasitic resistance rx is used for deciding the frequency of oscillation of the relaxation oscillator. While the resistance rx is relatively small its value is not constant. Thus, the operation of the electronic interface reported in Abuelma'atti et al., (“A current conveyor-based relaxation oscillator as a versatile electronic interface for capacitive and resistive sensors,” International Journal of Electronics, Vol. 92, 2005, pp. 473-477—incorporated herein by reference) may not be reliable.